1. Field of the Invention
This invention relates to an image processing method and a computer readable medium for image processing, for displaying an image using volume data such as a medical image.
2. Description of the Related Art
In recent years, attention has been focused on a technology of visualizing inside of a three-dimensional object with the progression of the image processing technology using a computer. Particularly, medical diagnosis using a CT (Computed Tomography) apparatus or an MRI (Magnetic Resonance Imaging) apparatus capable of visualizing inside of a living body to find a lesion at an early stage is widely conducted in medical field.
A method called volume rendering is known as a method of providing a three-dimensional image of the inside of an object. In the volume rendering, a virtual ray is projected to a three-dimensional volume data filled with voxel (minute volume element), whereby an image is projected onto a projection plane. A ray casting method is one kind of volume rendering. In the ray casting method, sampling is performed at given intervals along the ray path, the voxel value is acquired from the voxel at each sampling point, and color information and opacity are calculated from the voxel values.
Voxel is a unit of a three-dimensional region of an object, and the voxel value is unique data representing the characteristic of the density value, etc., of the voxel. The whole object is represented by voxel data, which is typically a three-dimensional array of the voxel values. Usually, two-dimensional tomographic image data provided by a CT apparatus are accumulated along the direction perpendicular to the tomographic plane, and necessary interpolation is performed, whereby voxel data of a three-dimensional array is provided.
Volume data and image data provided by processing the volume data are stored in a server or a client computer. The image stored in the computer is opened (namely, the image data stored in the computer is accessed), whereby the computer can read the image data and execute a predetermined image processing program for displaying the image in a user-operable state. Before the image is opened, a user can view a preview image of the image. In a certain system, as for an image not yet opened, a 2D (two-dimensional) slice image (cross-sectional image) is displayed as a preview image; as for an image once opened, an image in the last edit state (often, a 3D image: three-dimensional image) is displayed.
FIG. 13C shows a preview image when images of “patient D” and “heart 2” are selected out of menus shown in FIGS. 13A and 13B. This preview image is for an image once opened, and the image (heart 2) last displayed in a previous task by the user (in a last edit state) is displayed. The preview image is not made by rendering at that time. The preview image is only a copy of the image displayed on the screen as a result of the previous task and therefore its internal entity is a 2D image.
FIG. 14 shows an outline of a flow in a related art for opening an image. It is assumed that image data is stored in a server, etc., on a network. When the user completes an edit task of an image (S101), task state for the image is stored (step S102). When the task state is stored, the last image displayed on the monitor is stored (step S103). The task state and the last displayed image are transferred from the computer as the client to the server (step S108).
Next, the user selects an image (step S104). Then, the last displayed image (if the image is not yet opened, one slice image) is transferred from the server to the computer of the client (step S109). Accordingly, the last image displayed on the monitor (if the image is not yet opened, one slice image) is displayed as a preview image (step S105). When the user performs an operation to open the image with respect to the preview image (step S106), the task state and the image data are transferred from the server to the computer as the client (step S110). Accordingly, the image is opened using the stored task state (step S107). More than one task state may be generated for the same image data because a plurality of features involved in the same image data may be rendered under different conditions.
On the other hand, hitherto, fly through display has been used in performing virtual endoscope display. The fly through display is to display a moving image as if flying through a human body, by moving a view point along the previously generated path in performing virtual endoscope display.
FIG. 15 shows an example of the fly through display of a virtual endoscope image in a related art. FIG. 16 is a schematic representation of the fly through display in the related art. In an endoscopy in the related art, as shown in FIG. 16, projection is conducted from a view point 111 onto a projection plane 110 to generate an image as a display image 112 (step S201 in FIG. 15). In this case, the fly through display is a moving image and therefore a plurality of display images 112 is stored in time sequence (step S202). At the preview display time, the display images 112 are called and are displayed as a moving image (step S203).
Thus, for the preview image in the related art, if the image is an image opened in the previous task, only one image in the last edit state displayed on the screen is displayed. On the other hand, if the image is an image not yet opened, namely, an image whose image data is not read and for which a predetermined image processing program is not yet executed, only one 2D slice (cross-sectional image) is displayed.
Therefore, if a plurality of images of the same patient exists, if more than one task state exists for the same image, etc., the preview images resemble and if the preview images are provided at different angles, it becomes more difficult to distinguish the preview images from each other. Thus, it is hard to determine which image or which state related to the image is to be opened. Particularly, the size of the image data including volume data is large and it takes time from acceptance of an instruction to display the image until the image is displayed and therefore it may be difficult to open appropriate image data with good operability.
In the fly through display in the related art, when operating image data, the direction of the view point can be changed dynamically even while fly through display is produced as a moving image, and the region of interest can be displayed at an angle easy to be viewed.
However, when viewing the preview image of the fly through display in the related art, although a moving image with the view point moving on the path can be displayed, the view point is fixed to the direction specified at the moving image generation time. A field of view not included in the previously specified view point is not displayed as a preview, and therefore it may be difficult to select the objective image rapidly.